Use bolt hashmap freelist for metadata databases - #6943

bolt documents the array freelist as potentially degrading significantly
for large fragmented databases, while the hashmap freelist is faster in
almost all circumstances. From the [DB.FreelistType] GoDoc:

    // FreelistType sets the backend freelist type. There are two options. Array which is simple but endures
    // dramatic performance degradation if database is large and fragmentation in freelist is common.
    // The alternative one is using hashmap, it is faster in almost all circumstances
    // but it doesn't guarantee that it offers the smallest page id available. In normal case it is safe.
    // The default type is array
    FreelistType FreelistType

While the default is still `FreelistArrayType`, the package shows that
the intent is to make `FreelistMapType` the default in future;
https://pkg.go.dev/go.etcd.io/bbolt@v1.5.0#pkg-constants

    // TODO(ahrtr): eventually we should (step by step)
    //  1. default to `FreelistMapType`;
    //  2. remove the `FreelistArrayType`, do not export `FreelistMapType`
    //     and remove field `FreelistType' from both `DB` and `Options`;

This keeps the on-disk format unchanged and only changes the in-memory
freelist implementation used after opening the DB.

[DB.FreelistType]: https://pkg.go.dev/go.etcd.io/bbolt@v1.5.0#DB.FreelistType

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>